Median net worth increased by $61,379 for the top 20 percent of households, which more than doubled the share of their wealth relative to lower-income groups, the report found. For those in the bottom 20 percent, net assets minus liabilities declined by $5,124. U.S. median household net worth fell by more than $5,000, or 6.8 percent, from 2000 to 2011.

The result was a widening wealth gap between those at the top and those in the middle and bottom of the net-worth distribution, according to the report.

Net worth fell for all age groups younger than 55, according to the report. Between 2000 and 2011, median net worth increased for those 65 and older.

Concern over rising U.S. inequality flared this spring when French economist Thomas Piketty published research showing that returns on capital historically outstrip gains from economic growth. Last month, research by European Central Bank economist Philip Vermeulen and London School of Economics' Gabriel Zucman showed that the wealth of the super-affluent is undercounted. The top 0.1 percent of America's rich - those worth at least $20 million - held 23.5 percent of all U.S. wealth in 2012, they found.